The invention is applicable within different technical fields where an axial displacement of an object shall be provided. The invention is especially applicable in applications having great demand for speed and controllability of the axial displacement, as well as having great demand for low noise levels, such as the application to control a valve, e.g. an intake valve or an exhaust valve of a combustion engine.
An actuator, usually known as a pneumatic actuator, comprises an actuator piston that is displaceable in the axial direction in a reciprocating manner between a first position and a second position. The displacement is accomplished by means of controlling a pressure fluid, such as pressurized air, acting against the actuator piston, the actuator piston in its turn acting against the valve, or the like, in order to control the position thereof. There are also hydraulic actuators working in accordance with the same principles.
When the actuator piston is in its first/idle position the valve is in contact with its valve seat, and when the actuator piston is in its second/active position the valve is open, i.e. located at a distance from said valve seat.
The axial displacement of a conventional cam shaft controlled engine valve is controlled such that during closing of the engine valve the closing speed is decreased just before the engine valve abut the valve seat due to the shape of the cam, however, a pneumatically controlled engine valve will have accelerating or maximum closing speed at the time of contact with the valve seat. This entails that the engine valve, the valve seat and the actuator piston rod are subject to considerable wear. Thereto, the engine valve and actuator piston might rebound which create noise, vibrations and incorrect operation of the engine.
Thus, there is a demand for a decreased or limited closing speed just before the engine valve come in contact with the valve seat, in order to reduce rebound, noise and vibrations.
The prior art documents U.S. Pat. No. 6,192,841, U.S. Pat. No. 6,412,457 and U.S. Pat. No. 7,387,095 discloses actuators for engine valves having controlled/decreased closing speed at the time of impact, however, these actuators are from a technical view truly complicated and comprises numerous movable elements in order to accomplish the controlled/decreased closing speed.
The applicants own U.S. Pat. No. 7,121,237 discloses an actuator for axial displacement of an object.